Labour's new leader Jacinda Ardern and other opposition MPs will speak against the Government's Pay Equity Bill ahead of its anticipated first reading in parliament on Tuesday.
The Public Services Association (PSA) has organised the protest and speeches will be made in the Beehive.
The Bill, introduced last month, sets the rules around any future pay equity claims.
It follows on from the historic $2 billion care worker settlement that came into force on July 1.
Unions hailed that as a breakthrough, but they have serious issues with the way the Bill has been drafted and say it puts barriers in the way of women in other industries seeking the same pay equity deal.
"This legislation is not about pay equity, it is about trying to shut it down," said PSA national secretary Erin Polaczuk.
"We'll hear some strong criticisms from both unions and political parties about the scope and impact of this repressive legislation."
The PSA has previously said a key problem is that the Bill adds additional barriers by putting significant restrictions on male dominated jobs, which can be used as a comparator to help determine whether a job has been undervalued because it is female dominate.
Joining Ms Ardern at the rally will be Māori Party co-leader Marama Fox, Green Party MP Jan Logie and NZ First MP Tracey Martin.